A downhole measurement-while-drilling (“MWD”) tool that transmits measurement data uphole using electromagnetic (“EM”) telemetry includes an electrical insulation layer (e.g., ceramic, hard plastic, rubber) positioned between an upper portion of the tool and a lower portion of the tool. To transmit the data from within a wellbore to the surface, a coding method is used. For example, a predetermined carrier frequency is selected, and a PSK or QPSK coding is superposed to define a bit pattern. This coded signal is applied as a voltage differential between the upper and lower portions of the tool. Due to the voltage differential, current is generated that travels through the subterranean formation. More particularly, the current travels from the lower portion of the tool, out into the subterranean formation, and bends back toward the upper portion of the tool, in an almost semi-elliptical shape.
Two sensors (e.g., metallic stakes) are driven into the subterranean formation at the surface. When a portion of the current reaches the stakes, a voltage differential is generated across the stakes, as the surface formation has some electrical resistivity. A computer system at the surface then decodes the voltage differential to recover the measurement data that was transmitted from the downhole tool in the wellbore.
Equipment (e.g., electrical motors, generators, control equipment, etc.) located at the surface may be electrically-connected to a drill string and/or a casing in the wellbore. For example, the equipment may be electrically-connected to the drill string and/or the casing through metallic structural components that are part of the drilling rig. As a result, current from the equipment may be transmitted into the wellbore via the drill string and/or the casing. This current may be introduced into the subterranean formation and affect (e.g., interfere with) the telemetry measurements obtained by the stakes.